In recent years, there has been an increasing tendency to use liquid crystal displays (LCD's), dot matrix displays, and other flat displays in modern avionics. Such devices typically offer the advantages of long life, lower power consumption, high resolution and definition, and multi-colored displays.
At the same time, it is necessary to back-light the display in order that its indicia and information may be seen against a contrasting background. To date, several back-lighting techniques have been developed. These techniques include the use of fluorescent illumination, electro-luminescent panels, incandescent lighting and ganged light-emitting diodes (LED's). Each of these prior art techniques is believed to have individual disadvantages and shortcomings.
For example, fluorescent lamps must be operated continuously in order to back-light the display. This causes considerable heat to be generated. Fluorescent lamps are also temperature-dependent, particularly during start-up conditions. The light output of such lamps may vary by a factor of about 100 within an operating range of from about -20.degree. C. to about +40.degree. C. During cold start-up conditions, considerable heat is required to initially vaporize the mercury, and to break down the vapor into a self-maintaining discharge. This discharge, which is rich in ultraviolet radiation, excites a visible radiation from a phosphor or fluorescent coating on the inside of the tube. The particular wavelength of light generated by mercury vapor (i.e., .lambda..sub.Hg =254 nanometers) is believed to destabilize the silicon transistor matrix in the LCD. Another problem is that fluorescent lamps are usually formed as elongated tubes. Hence, it is necessary to diffuse the light from such tubes to uniformly illuminate a large area behind the LCD display. While the efficiency of the phosphor used in fluorescent lamps is typically on the order of about 80 lumens per watt, such tubes normally have a maximum output of about 6,000 foot-Lamberts (ft-L). In passing through the diffuser and the LCD display itself, however, the intensity of light available for usable display contrast may be dramatically reduced to about 200 (ft-L). While this level may be acceptable under normal room conditions, under conditions of brilliant sunshine, such as in the cockpit of an aircraft, the ambient light intensity may be on the order of about 10,000 ft-L, thereby making the display difficult to read. In effect, a high level of ambient light may literally "wash out" the normal contrast between the displayed information and the background illumination. Additional details of such fluorescent back-lighting techniques may be found in Mercer and Schoke, "Fluorescent Backlights for LCDs", Information Display at pp. 8-13 (Nov. 1989), and Kishimoto and Terada, "Flat Fluorescent Lamp for LCD Back-Lighting", SPIE, Vol. 1117, Display Systems Optics II at pp. 168-176 (1989 ).
It is also known to use electro-luminescent panels to back-light an LCD display. With such panels, the problem of non-uniformity is minimal. However, two other problems become evident. Such panels are considerably less bright than fluorescent tubes. Luminances on the order of about 30 ft-L are commonly reported. Secondly, these panels are also temperature-dependent, and it is necessary to heat the panel in order to maintain even limited brightness. As much as 17 watts per square inch [2.635 watts/cm.sup.2 ] of power may be required during cold starts. Moreover, the amount of light generated decreases over time. With some panels, light output is expected to decrease by about fifty percent after about 1500 hours of use. Additional details of such electro-luminescent panels may be found in U.S. Pat. No. 4,767,965 ("Flat Luminescent Lamp for Liquid Crystalline Display"), and U.S. Pat. No. 4,143,404 ("Laminated Filter-Electro-luminescent Rectifier Index for Cathode Ray Display").
Incandescent lamps have also been used to back-light an LCD display. However, non-uniformity of illumination is a common problem. Moreover, these lamps are relatively inefficient, as compared with fluorescent tubes, and usable life is somewhat limited. As a result, incandescent lamps are not believed to be in common use for back-lighting LCD's.
Finally, ganged LED's have also been used as back-light sources. Here again, uniformity of illumination is a persistent problem, typically requiring the use of a diffuser. Moreover, power consumption is typically greater than with fluorescent tubes and electro-luminescent panels.
Accordingly, there is believed to be a need for an improved means for back-lighting an LCD or dot matrix display, which affords the advantage of high-contrast with the LCD under extreme conditions of ambient lighting, which has a controllable brightness, which is reliable, which affords uniform illumination of the display, which has a long service life, and which does not require heating.